用于小动物脑磁共振成像及功能磁共振成像的正交微波传输带线圈设计

目的 提出一种适用于小动物整个头部成像的高场射频线圈设计方法,以解决表面线圈在脑深部区域信噪比低的问题.方法 采用微波传输带结构,并集成了猫头部固定系统.根据设计结果制作的原型线圈由16个微波传输带单元构成.分别采集了矿物油、水模以及猫脑图像验证线圈性能.结果 猫脑感兴趣区域内信噪比及敏感度分布均匀;功能磁共振成像结果清楚显示了连接外侧膝状核及初级视皮层的丘脑皮层视觉网络.结论 MTL线圈的动物固定系统能够稳定地固定猫头部,减小运动伪影,提高图像质量;该线圈适用于脑功能磁共振研究,尤其是脑深部区域的fMRI研究.     

常规磁共振对裸鼠成像的可行性及参数优化

目的探讨临床常规磁共振在裸鼠成像中的可行性。方法采用小关节线圈及专用小动物线圈对裸鼠肝脏进行MR成像,根据伪影、均匀度与对比度,评价两种线圈对裸鼠成像的质量。对裸鼠进行T1mapping和T2mapping成像,根据肝脏的T1、T2值确定扫描序列中的TE值,然后采用浮动的TR值进行扫描,最后调整扫描的其它参数实现序列的优化。结果在FSE T1WI、SE T1WI、FSE T2WI、GRE T2*WI序列中,小动物线圈和3英寸双环形线圈的图像质量评分分别为2.6±0.54、3.2±0.84、2.6±0.89、2.4±0.55和3.6±0.55、3.8±0.45、3.2±0.84、2.6±0.55,小动物线圈图像的信噪比明显高于3英寸双环形线圈(P0.05)。经过序列优化后,图像的细节分辨率,对比度明显提高,图像信号显示更均匀。结论通过线圈的优选及序列的优化,常规临床磁共振可以进行活体裸鼠磁共振成像。     

初步探讨磁共振显微线圈在眼眶软组织肿块诊断中的应用价值

目的：对比16通道头颅线圈及显微线圈眼部软组织肿块磁共振成像特征,初步探讨磁共振显微线圈在眼科的临床应用价值.材料与方法：本次研究中,分别应用16通道头颅线圈及显微线圈,对20例临床疑是眼眶内占位性病变的患者进行磁共振眼部成像.所有图像采集均应用1.5T磁共振扫描仪,接收线圈为显微线圈（直径47mm）及常规头颅线圈.对病灶的定性分析包括对肿块的数量、大小、位置、范围、边界、形态学细节、生长方式及其与周边组织间关系的评估.对病灶的定量分析主要是对比两种线圈成像信噪比（SNR）.结果：对比16通道头颅线圈,磁共振显微线圈能清晰展示肿块与周边组织间的关系（z=-4.3589）、肿块的内部特征（z=-4.3589）、肿块的范围及其边界（z=-4.1231）,研究结果基本与术中观察一致.此外,那些不容易被常规头颅表面线圈检出的小肿块,较易被显微线圈检出（n=5）.显微线圈对病变显示的图像信噪比（SNR）明显高于常规头颅表面线圈（z--3.9199）.结论：磁共振显微线圈的应用,有利于正确评估眼部软组织肿块及其与周边组织的关系,为手术术式及治疗方案的选择提供一些客观证据.     

利用颅脑射频线圈进行大鼠磁共振成像

目的 研究3.0T磁共振成像系统中大鼠脑部射频线圈.方法 提出一种设计线圈结构的方案,采用高等于直径的鞍型线圈,研究直径为5 cm的大鼠脑部线圈减小电容和分布电容,可使线圈带宽减小,提高线圈品质的因素(Q).将线圈与人体头部线圈和体部线圈分别对自制的模型利用同一序列进行扫描,对三组图像选择同一位置的图像,比较各线圈的信噪比(SNR).观察图像的质量,应用大鼠颅脑模型分别进行轴位、矢状位和冠状位T1W FLAIR或T2W扫描.结果 线圈的SNR比现有的头部线圈高5倍以上.大鼠图像能很好地显示脑室结构,可清楚分辨脑部的灰质和白质.结论 利用所设计的线圈可获得具有很高SNR的图像,在大鼠脑部影像研究中取得了很好的效果.     

基于平稳小波变换的减小MR图像截断伪影的方法

如何缩短磁共振成像的时间一直是磁共振成像领域研究的热点问题之一.在实际工作中只采集部分相位编码的磁共振信号进行磁共振图像重建,不但可以缩短成像的时间,而且可以使图像保持较高的信噪比,但在重建的图像上出现了截断伪影.采用平稳小波变换和图像模板技术消除图像截断伪影及噪声,可有效地提高图像质量.     

磁共振图像特征及其对比机制(一):信号、噪声及信噪比

<正>信噪比是科学与工程领域常用的参数,也是医学影像学领域常用的评价图像质量的重要指标之一。本文主要探讨MRI成像系统中信号以及噪声来源,信噪比及其影响因素等。MRI中的信号根据法拉第电磁感应定律,当射频激发停止后旋转的横向磁化矢量M_(xy)(t)于接收线圈内感应     

表面线圈接受信号对磁共振波谱化学位移成像分析的影响

目的探讨表面线圈接受信号对磁共振波谱分析的影响.方法实验研究采用含有0.1% Gd-DTPA(0.5 mmol/ml)的500 ml 生理盐水模型:一是将其直接放置在柔性线圈表面上,横断面、冠状面T1加权成像,然后对中心层面图像Profile测定;MR检查后对同一模型进行多体素化学位移成像氢波谱检查.二是将同一模型包绕在线圈内,采用上述同样方法进行检查.另对1例人体肩背部脂肪瘤进行活体多体素化学位移成像氢波谱检查,柔性表面线圈贴近肩背部病灶部位进行信号采集.结果水模型直接放在表面线圈上,等中心层面横断面图像Profile显示中心明显低于左右两侧,远离线圈侧明显低于靠近线圈侧,波谱亦显示感兴趣区内远离线圈侧波谱信号幅度明显低于靠近线圈侧.水模型包绕在线圈内,等中心层面横断面图像Profile在左右、前后方向基本均匀分布;MRS波谱信号幅度大小分布与距离线圈远近无关.水模型冠状面Profile显示靠近线圈中心明显高于两侧.人体脂肪瘤波谱图像示感兴趣区内靠近表面线圈侧波谱信号幅度明显高于远离线圈侧.结论表面线圈接受信号不均匀性会对磁共振波谱定量分析产生直接影响;将检查对象放置在线圈中心并包绕在柔性线圈内可以基本排除其对波谱定量分析的影响.     

超高场MRI：脑部成像结果探讨

随着射频和线圈技术的提高,超高场磁共振近年来有了很大的发展。与常规的场强,超高场MR主要具有以下优势：（1）信噪比的显著增加保证了高分辨率和高质量的图像,从而大大提高了微小结构的检出;（2）由于磁敏感效应的增加,T2＊或磁敏感技术有了更广泛的应用,尤其是对异常的铁沉积,微小出血点和小静脉血管的检测;（3）高场T1弛豫时间的增加可以提高ASL灌注成像技术的应用;（4）信号本身的增加也可提高fMRI和MRS的分辨率等。当然,目前超高场强磁共振技术也还有一些局限性：（1）SAR的明显增加限制了采集层数并影响采集时间;（2）高场下RF磁场（B1）的不均匀性造成图像信号的不均匀;（3）增加的磁敏感效应也在颅底增加了相应的伪影等。本文对高场7T在纽约大学医疗中心脑部MR的最初临床应用结果 进行了分析。     

可穿戴式磁共振设备头部柔性和弹性线圈设计及验证

目的为了减少高场磁共振成像中因患者身体几何尺寸的差异而引起的扫描部位与线圈尺寸的匹配程度对信噪比(signal-to-noise ratio,SNR)的影响.材料与方法选用成人头部为研究对象,采用共边正六边形填充线圈表面的方式制作柔性头线圈.在验证自制线圈与商用线圈的性能时,分别用水膜和真实人体头部开展对比实验.结果水...     

数字的转换,跨越的时代--飞利浦全数字磁共振Ingenia突破性技能分析

与CT、DR等全数字影像设备不同,目前市场上的磁共振产品仍旧由线圈采集图像信号,再将模拟信号通过模拟线圈接口与同轴电缆传输到射频通道进行A-D模数转换,获取最终数字图像.这种非全数字传输方式存在信息损失、电子线路间干扰或电路复杂等弊端,会对终端结果产生一定影响.2012年,飞利浦率先推出全数字磁共振系统Ingenia(图1),预示"磁共振模拟或半数字时代"的结束.     

Radiofrequency coils for musculoskeletal magnetic resonance imaging

Dedicated and specialized radiofrequency coils are critical for high quality musculoskeletal magnetic resonance imaging (MRI). Dedicated coils improve the signal to noise ratio, allowing for faster or higher resolution examinations. Transmit-receive coils can reduce heating at high field strength. Finally, novel radiofrequency coils can be used for assessment of tissue biochemistry, as seen with sodium MRI.     

Flow, radiofrequency pulse sequences, and gradient magnetic fields: basic interactions and adaptations to angiographic imaging

The basic process of MRI consists of two essential, relatively independent components: (1) excitation in the form of a radiofrequency pulse sequence, and (2) signal sampling and localization, that is, forming the MR image through the use of field gradients. The presence of motion (blood flow) during either excitation or sampling results in two types of corresponding effects: (1) time-of-flight effects, and (2) spin phase phenomena. These effects can be manipulated through the use of special coils, pulse sequences, gradients, and postprocessing techniques to provide angiographic images in which simple motion provides the basis for contrast.     

MR-guided endovascular interventions: device visualization, tracking, navigation, clinical applications, and safety aspects

Reliable visualization and tracking are essential for guiding endovascular devices within blood vessels. The most commonly used methods are susceptibility artifact-based tracking that relies on the artifact created within the image by the device and microcoil- or antenna-based tracking that uses the high signal generated by small MR endovascular receive coils when the transmit coil emits a nonselective radiofrequency pulse. To date, the use of endovascular MR guidance techniques has primarily been confined to animal experiments. There are only a few reports on MR-guided endovascular applications in patients. Therefore, access to the patient within the scanner, dedicated devices, and safety issues remain major challenges. To face these challenges, attention from all radiologists, especially interventional radiologists, is required to make MR-guided endovascular procedures a clinical reality.     

Birdcage coils: Equivalent capacitance and equivalent inductance

Birdcage coil is extensively used in MR systems thanks to its possibility to provide high signal‐to‐ noise ratio and high radiofrequency magnetic field homogeneity that guarantee a large field of view. This work describes how to schematize the birdcage coil in terms of an equivalent inductance and an equivalent capacitance, whose knowledge can be useful for coil design and characterization. In particular, the knowledge of equivalent capacitance and equivalent inductance permits to estimate theoretically coil resonant frequency, quality factors and matching circuit capacitor values in a quick way, while workbench tests permit to estimate coil resistance and sample‐induced resistance. The presented theory is validated for both lowpass and highpass birdcage coils.by using literature data. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 44B: 32–38, 2014     

Numerical methods and software tools for simulation,design, and resonant mode analysis of radio frequency birdcage coils used in MRI

Design of magnetic resonance imaging (MRI) radiofrequency (RF) coils using lumped circuit modeling based techniques begins to fail at high frequencies, and therefore more accurate models based on the electromagnetic field calculations must be used. Field calculations are also necessary to understand the interactions between the RF field and the subject inside the coil. Furthermore, observing the resonance behavior of the coil and the fields at the resonance frequencies have importance for design and analysis. In this study, finite element method (FEM) based methods have been proposed for accurate time‐harmonic electromagnetic simulations, estimation of the tuning capacitors on the rungs or end rings, and the resonant mode analysis of the birdcage coils. Capacitance estimation was achieved by maximizing the magnitude of the port impedance at the desired frequency while simultaneously minimizing the variance of RF magnetic field in the region of interest. In order for the proposed methods to be conveniently applicable, two software tools, resonant mode and frequency domain analyzer (RM‐FDA) and Optimum Capacitance Finder (OptiCF), were developed. Simulation results for the validation and verification of the software tools are provided for different cases including human head simulations. Additionally, two handmade birdcage coils (low‐pass and high‐pass) were built and resonance mode measurements were made. Results of the software tools are compared with the measurement results as well as with the results of the lumped circuit modeling based method. It has been shown that the proposed software tools can be used for accurate simulation and design of birdcage coils. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 13–32, 2015     

Visualization, tracking, and navigation of instruments for magnetic resonance imaging-guided endovascular procedures

Interventional procedures using magnetic resonance imaging (MRI) guidance have increased in interest during the last few years. Central to the success and safety of MRI-guided procedures is the accurate visualization of the interventional instruments relative to the surrounding anatomy. A variety of methodologies for visualizing and automatically tracking instruments, including needles, radiofrequency and laser ablation devices, endoscopes, catheters, and guidewires have been developed and introduced to help the interventionalist to safely guide the device toward the target region. This article describes and compares characteristics of the four most commonly used localization and tracking systems used for MRI-guided interventional procedures: those based on the susceptibility artifact of the device, those that intentionally create field inhomogeneity along the device, those that rely on an optical tracking system, and active tracking systems using micro receive coils.     

Perspectives on body MR imaging at ultrahigh field

As investigators consider approaching the challenge of MR imaging at field strengths above 3T, do they follow the same paradigm, and continue to work around the same problems they have encountered thus far at 3T, or do they explore other ways of answering the clinical questions more effectively and more comprehensively? The most immediate problems of imaging at ultrahigh field strength are not unfamiliar, as many of them are still pressing issues at 3T: radiofrequency coils, B1 homogeneity, specific absorption rate, safety, B0 field homogeneity, alterations in tissue contrast, and chemical shift. In this article, these issues are briefly reviewed in terms of how they may affect image quality at field strengths beyond 3T. The authors propose various approaches to overcoming the challenges, and discuss potential applications of ultrahigh field MR imaging as it applies to specific abdominal, pelvic, peripheral vascular, and breast imaging protocols.     

Intravascular MRI

In patients with vascular disease, acute coronary syndromes and ischemic strokes develop suddenly and often unpredictably. In most patients, these clinical scenarios result from arterial thrombosis from one of three mechanisms: plaque rupture, plaque erosion, or calcified nodule. A number of diagnostic modalities have been used in the evaluation of these unstable, high-risk lesions that predispose to arterial thrombosis. Noninvasive MRI allows three-dimensional imaging with evaluation of vascular structures and depiction of components of atherosclerotic plaque. However, noninvasive MRI is limited in the evaluation of arteries of smaller caliber and deeper location, such as coronary, iliac, and renal arteries. To overcome these inherent limitations of noninvasive MRI, invasive approaches have been developed that include intravascular coils for lesion assessment and characterization, and a novel intravascular MRI catheter within which the magnets, radiofrequency transmitters, and receivers are miniaturized. This self-contained MRI catheter holds promise in the in vivo assessment of lipid-rich, potentially vulnerable plaques.